water tank overflows not ball cock or split coil in cylinder

[dannyabb]

I have a problem with water flowing out of my overflow pipe. Ball cock is not the problem as i have isolated it. The exess water only flows when the hot water comes on (not radiators just hot water), it comeing in from the bottom pipe in the tank (the one that should fill the system) and the water in the tank is warm. I have put a new water cylinder in and am getting the same problem. Have tested it over a couple of hours when the hot water was on and the water is emptying from the larger tank in the loft for the water cylinder.

CAN ANYBODY PLEASE HELP

One more thing i do not have a mixer shower but i do have a tap in the kitchen if that helps.

you say the large tank is emptying.

is it the small tank that is filling with warm water ?

[dannyabb]

yes it is

faulty coil

in the airing cupboard should be a gate valve that you can shut to stop the supply of cold water from header tank to the base of the cylinder.
shut that and does the large tank stop emptying and small tank filling ?

[dannyabb]

i tryed this with the old tank and that was the case so i changed the cylinder thinking it was the coil in it. but as i only put the new cylinder in yesterday i would have thought it would have solved it

never seen a new cylinder fail like that but even new are not always 100%.

the coil is the only means that your dhw from the cylinder can cross to heating water.

thats the only part of the system the two come into contact.

[dannyabb]

so it looks like it is a faulty one them. thanks for the help

have you still got the old cylinder ?

[dannyabb]

yes i have

take it in the garden lay it on its side and run the hose through the coil.
after a while you should see it leaking water into the cylinder.

you didn't twist the coil inlets when tightening the joints on the new cylinder did you ?